归并求逆序对

主要步骤是在对子序列S1,S2进行归并时，若S1[i]>S2[j],则逆序数加上 s1.length-i

#include <iostream>#include <vector>using namespace std;void Merge(vector<int> &vec,size_t p,size_t q,size_t r,int &sum){auto n1=q-p+1;auto n2=r-q;vector<int> lhs(n1+1);vector<int> rhs(n2+1);for(int i=0;i<n1;++i)lhs[i]=vec[p+i];for(int j=0;j<n2;++j)rhs[j]=vec[q+j+1];    //哨兵lhs[n1]=100000000;   rhs[n2]=100000000;int i=0;int j=0;for(size_t k=p;k<=r;++k){if(lhs[i]<=rhs[j]){vec[k]=lhs[i];i++;}else{vec[k]=rhs[j];j++;if (lhs[i]!=100000000)sum=sum+n1-i;   //主要步骤，其余同归并排序}}for(auto v:vec)cout<<v<<" ";cout<<sum<<endl;}void mergeSort(vector<int> &vec,size_t p,size_t r,int &sum){if(p<r){size_t q=(p+r)/2;mergeSort(vec,p,q,sum);mergeSort(vec,q+1,r,sum);Merge(vec,p,q,r,sum);}}int main(){vector<int> vec{5,2,4,7,1,3,2,6};int sum;mergeSort(vec,0,vec.size()-1,sum);for(auto v:vec)cout<<v<<" ";cout<<endl;cout<<sum<<endl;return 0;}